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INTERCLAVICLE

33

piece,   apparently   comparable   to   a   xiphoid   process,   has   been
detected.

Fia. 20.—Sternum of Radolpbi's Whale (Hal-
aenoptera boreaUs), showing its relation to
the inferior extremities of the first pair of
ribs. x -fa. (From Flower's Osteology>}

FIG. 21.—Sternum of a yomig
Dugong (Halicwe indicus}.
x J. From a specimen iu
the Leydeu Museum. j?>5,
Presternum ; aw, xiphister-
nuui. (From Flower's Oste-
ology).

From the instances which have been described, as well as
from the mode of- development of the sternum and from the
number of free ribs, i.e, ribs which are not attached to it, it
would seem that the sternum, has undergone a considerable reduc-
tion in its size. This reduction may be possibly accounted for
by the need for respiratory activity, which is clearly increased by
a less-marked fixity of the walls of the thoracic cavity. In the
case of the "Whales one can hardly help coming to that conclusion.
The arrangement in the Monotremata does not, however, point in
the same direction; for these animals are precisely like the
higher Mammalia in the reduction of the sternum and of the
number of ribs which reach it.
The JEpisternum.—The Mammalia are as a rule to be dis-
tinguished from lower Vertebrates by the absence of ail episternum,
or iiiterclavicle as it is also called. In the Monotremata, however,
there is a large T-shaped, bone which does not overlie the ster-
num as in reptiles, but is anterior to it. The relations of this
bone to the clavicles seem to leave no doubt that it-is the
equivalent of the Lacertilian interclavicle or episternum. The
Monotremata are not, however, the only mammals in which this
structure is to "be seen. The Mole in the embryonic condition is
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